Transactions and Scripts: DUP HASH160 ... EQUALVERIFY CHECKSIG

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The nature of Bitcoin is such that once version 0.1 was released, the core design was set in stone for the rest of its lifetime.  Because of that, I wanted to design it to support every possible transaction type I could think of.  The problem was, each thing required special support code and data fields whether it was used or not, and only covered one special case at a time.  It would have been an explosion of special cases.  The solution was script, which generalizes the problem so transacting parties can describe their transaction as a predicate that the node network evaluates.  The nodes only need to understand the transaction to the extent of evaluating whether the sender's conditions are met.

The script is actually a predicate.  It's just an equation that evaluates to true or false.  Predicate is a long and unfamiliar word so I called it script.

The receiver of a payment does a template match on the script.  Currently, receivers only accept two templates: direct payment and bitcoin address.  Future versions can add templates for more transaction types and nodes running that version or higher will be able to receive them.  All versions of nodes in the network can verify and process any new transactions into blocks, even though they may not know how to read them.

The design supports a tremendous variety of possible transaction types that I designed years ago.  Escrow transactions, bonded contracts, third party arbitration, multi-party signature, etc.  If Bitcoin catches on in a big way, these are things we'll want to explore in the future, but they all had to be designed at the beginning to make sure they would be possible later.

I don't believe a second, compatible implementation of Bitcoin will ever be a good idea.  So much of the design depends on all nodes getting exactly identical results in lockstep that a second implementation would be a menace to the network.  The MIT license is compatible with all other licenses and commercial uses, so there is no need to rewrite it from a licensing standpoint.
A second version would be a massive development and maintenance hassle for me.  It's hard enough maintaining backward compatibility while upgrading the network without a second version locking things in.  If the second version screwed up, the user experience would reflect badly on both, although it would at least reinforce to users the importance of staying with the official version.  If someone was getting ready to fork a second version, I would have to air a lot of disclaimers about the risks of using a minority version.  This is a design where the majority version wins if there's any disagreement, and that can be pretty ugly for the minority version and I'd rather not go into it, and I don't have to as long as there's only one version.

I know, most developers don't like their software forked, but I have real technical reasons in this case.

I admire the flexibility of the scripts-in-a-transaction scheme, but my evil little mind immediately starts to think of ways I might abuse it.  I could encode all sorts of interesting information in the TxOut script, and if non-hacked clients validated-and-then-ignored those transactions it would be a useful covert broadcast communication channel.

That's a cool feature until it gets popular and somebody decides it would be fun to flood the payment network with millions of transactions to transfer the latest Lady Gaga video to all their friends...
That's one of the reasons for transaction fees.  There are other things we can do if necessary.

How long have you been working on this design Satoshi?  It seems very well thought out, not the kind of thing you just sit down and code up without doing a lot of brainstorming and discussion on it first.  Everyone has the obvious questions looking for holes in it but it is holding up well Smiley
Since 2007.  At some point I became convinced there was a way to do this without any trust required at all and couldn't resist to keep thinking about it.  Much more of the work was designing than coding.

Fortunately, so far all the issues raised have been things I previously considered and planned for.